Natural killer T cells (NKTs) represent a subset of T lymphocytes with unique properties, including reactivity for natural or synthetic glycolipids presented by CD1d and expression of an invariant T cell antigen receptor (TCR) alpha chain. NKTs are different from functionally differentiated conventional αβ T cells in that they share properties of both natural killer cells and T cells are can rapidly produce both TH1-type and TH2-type responses upon stimulation with their ligands (innate immunity). The activation of NKTs paradoxically can lead either to suppression or stimulation of immune responses. For example, the production of TH1 cytokines is thought to promote cellular immunity with antitumor, antiviral/antibacterial, and adjuvant activities, whereas TH2 cytokine production is thought to subdue autoimmune diseases and promote antibody production. Because NKTs play a regulatory role in the immune system, they are attractive targets for immunotherapy.